There is an insulated type DC-DC converter as a circuit converting an input direct-current voltage to output a direct-current voltage having potential different from that of the input direct-current voltage by the use of a transformer. Moreover, the insulated type DC-DC converter includes one using a diode for rectifying a current flowing out of the secondary side coil thereof and one using a switching element. Between them, the one using the diode for rectifying a current has a simple circuit configuration, but has the problem of the generation of a rectification loss Vf·I caused by the forward voltage Vf of the diode and the current I flowing through the diode.
Accordingly, there is also a known synchronous rectification type DC-DC converter performing the rectification thereof by replacing the diode with a switching element (e.g. a transistor such as a MOS FET) having a small on-resistance to perform the on-off control of the switching element by synchronous control.
Moreover, an invention for achieving the reduction of power consumption at the time of a light load in an insulated type DC-DC converter was proposed (see, for example, Patent Document 1). The invention stops the operation of the control circuit controlling the switching element on the primary side. The switching loss caused by an on-off operation of a switching element is relatively large at the time of a light load of a switching power supply circuit. The invention of Patent Document 1 attains the further reduction of the power consumption by also stopping the control circuit generating on-off control signals as well as by reducing the switching loss.
Patent Publication 1: Japanese Laid-Open Publication No. 2002-233146